The present invention relates to a control rod drive for a nuclear reactor.
A conventional nuclear reactor typically has a plurality of fuel assemblies located in a reactor core, said fuel assembly having fuel rods each of which is composed of a plurality of fuel elements of fissionable material and a clad tube containing the fuel elements therein and made of anticorrosion material of high thermal conductivity. The reactor core comprises the fuel assemblies spaced from each other and control elements in the form of a rod or plate disposed between the fuel assemblies, the control elements being able to insert thereinto and extract therefrom. The reactor core is contained in a vessel so that coolant flows through the core and between the vessel and the core.
Now, .sup.235 U emits fast neutrons during its fission. As a nature of a neutron, it becomes easily absorbed by .sup.235 U with decrease of speed of the neutron, promoting fission of .sup.235 U. The successive fission is called "sustaining reaction". The sustaining reaction is controlled by use of moderators for fast neutrons and control elements for absorbing neutrons. A power of the reactor is in proportion to the neutron density.
If under the presence of such moderators and control elements one neutron emitted by fission causes the next fission producing a certain average number of new neutrons, the ratio of the numbers between the old neutron and the new neutrons is called an "effective multiplication factor k". With k = 1, the nuclear reactor can continue fission without supply of neutrons, and thus this is called a "critical condition".
When it is required to increase the power of a nuclear reactor, the control elements are extracted out of the reactor core so as to increase the multiplication factor above 1, while when it is required to decrease the power the control elements are inserted into the core so as to decrease the factor below 1. In either case, as soon as the power reaches a desired level, the control elements are moved to the position where the reactor is in a critical condition.
When an accident to a nuclear reactor occurs, the control elements are rapidly inserted into the core. In this case, there is no danger of rapidly cooling fuel elements and clad tubes even if the power decreases rapidly, because the reactor core has a sufficient heat capacity. Therefore, no adverse thermal effect due to the rapid insertion of control elements occurs.
On the other hand, if the control elements are rapidly extracted out of the core, the neutron density in the reactor core increases rapidly and the power also increases rapidly, which leads to deformation of fuel elements and clad tubes and interference therebetween due to the deformation, which further results in break down of the fuel elements and clad tubes and leakage of neutrons out of the reacter core which causes radioactive contamination.
A conventional control rod drive comprises a hydraulically driven piston and an index tube connected at one end with the piston and coupled at the other end with a control rod. The index tube is formed with a plurality of ringed grooves of equal pitch on the outer circumferential surface thereof. These ringed grooves cooperate with collet fingers which are constructed to be urged against the index tube and have means for enabling the collet fingers to engage with or disengage from the ringed grooves during movement of the index tube in the longitudinal direction thereof. The ringed grooves and collet fingers are shaped such that the collet fingers are able to disengage from the groove with ease when the index tube is moved toward the core, while the collet fingers lock the index tube preventing downward movement thereof when the index tube is moved in the opposite direction from the core.
When a control rod is inserted into the reactor core, the piston is driven toward the core and the collet fingers disengage from the ringed grooves and then engage with the next adjacent ringed grooves. This motion is repeated with the collet fingers sliding on the index tube. Therefore, it is possible to insert the control rod rapidly into the core for one pitch or notch of the ringed grooves or for several notches.
When the control rod is extracted out of the core, the index tube is moved toward the core for about half a notch disengaging the collet fingers out of the ringed grooves, so that the collet fingers contact the outer circumferential surface of the index tube. The collet fingers are in turn separated from the index tube by the collet finger separating means and then the index tube is driven in the reverse direction to the core for about one and half a notches. Thereafter, the collet fingers are released from the separating means and put on the index tube in engagement with the next ringed grooves.
It should be noted that there is a limitation in driving speed of the index tube because of a hydraulic mechanism for driving the piston and it is about 15 cm/sec at slowest. For this reason, the linear power density of the fuel rods located adjacent to the tips of the control rods increases about 10 KW/ft. This results in producing various drawbacks. In order to overcome this problem, an attempt has been made where the length of a notch of the grooves is shortened to increase engaging time. However, this has not been practical because it makes control rod extracting operations very complicated and maintenance very hard.